Within the framework of precision agriculture, there is the need to test harvested product for certain properties while still in the field. For example, the measured values can be electronically plotted to provide a fertilizer application map matched to the product properties or can be used to establish the market price of the harvest or to distribute the harvest into different containers in dependence on quality. In a combine, for example, the moisture content of harvested grain or its protein content can be determined. A measurement method that is available in particular is near infrared spectroscopy, in which a sample of the harvested grain is transported to a measurement chamber by gravity or an attached conveyor and irradiated there with broadband light, the spectrum of which (also) extends into the near infrared region. The light transmitted by or reflected from the sample is acquired by a detector and analyzed by the detector in dependence on wavelength. In this regard, see, for example, DE 10 2010 062 417 A1 and the references cited there.
Since calibration measurements are available from laboratory analyses, transmission measurement is preferred in many cases over a reflection measurement. To be sure, the average free wavelength of light within the grain sample is highly dependent on the harvested product, being, for example, about 9 mm in the case of rapeseed (canola) and about 20 mm for maize or soy. This means that it is advantageous to match the distance between the light source and the detector to the harvested product. For this, it was proposed in the prior art to make two opposite walls of the measurement chamber movable relative to each other and to equip one of the walls with the light source and the other with the detector (U.S. Pat. No. 6,559,655 B1) or to affix the light source to one wall and to mount the detector movably on the opposite wall, so that it can be moved more or less further from its wall into the measurement chamber, either by hand or by means of a motor (for comparison see WO 2007/034530 A2, which is seen as generic).
A problem in testing harvested product in a measurement chamber is that in the chamber the product can form bridges or cause a blockage or jamming there even when the size of the passageway in the measurement chamber (as described in WO 2007/034530 A2) is greater than the distance between the light source and detector is supposed to be. To avoid this problem, in the case of a moisture detector, separate elements were proposed for forced cleaning or emptying of the measurement chamber (DE 197 44 485 A1), which however were quite expensive. The use of the transmission principle, moreover, prevents a conveyor feeding the harvested product to the measurement chamber from extending into the measurement chamber to avoid jamming, since then it would adversely affect the measurement.
The problem underlying the invention will be seen in making available a measurement device that is an improvement over the prior art, and which does not have the said disadvantages or has them to a lesser degree.